1. Field of the Disclosure
The present disclosure relates to a planar lighting device used as a lighting unit for liquid crystal display devices or the like.
2. Description of the Related Art
Today liquid crystal display devices are commonly used as display devices of electronic devices such as personal computers and cellular phones. Since liquid crystal is not a spontaneous light emitting display element, for example, a transmission type liquid crystal display device requires a lighting unit for irradiating its liquid crystal panel, and a semi-transmission type liquid crystal display device using external light also requires an auxiliary lighting unit for using in a dark place. Planar lighting devices including a light-guiding plate and a light source arranged at the side of the light-guiding plate as principal components are widely used in combination with liquid crystal display devices as such a lighting unit for liquid crystal display devices, because of having the advantage of being easily reduced in thickness. Along with recent growing performance of white light-emitting diodes (LEDs), planar lighting devices using the white LEDs as light sources have also become commonplace for the purpose of further miniaturization, reduction in thickness, and reduced power consumption of planar lighting devices.
A conventional example of such planar lighting devices will be described with reference to FIG. 13. FIG. 13 illustrates the dimensions and positional relation of components for convenience. Although the components are illustrated with gaps therebetween, the components are appropriately arranged so as to be in intimate contact with each other or fixed to each other with double-sided adhesive tape, by fitting, or the like so as to achieve the most suitable optical characteristic in practice. This planar lighting device 10 includes a light-guiding plate 12 with a rectangular shape in a plan view and an LED 14 serving as a point light source arranged facing a light incident end face 12c that is one side end face of the light-guiding plate 12, and the illustrated example includes a housing frame 16 for housing these components. The light-guiding plate 12 includes a facing end face 12d facing the light incident end face 12c and a pair of principal faces 12a and 12b that couple the light incident end face 12c and the facing end face 12d with each other, with one principal face 12a (a front face) as a light exit face. The light-guiding plate 12 is formed by molding a transparent resin material such as an acrylic resin. Examples of the LED 14 include a white LED having a structure in which a blue light-emitting LED chip is sealed with a translucent resin in which yttrium-aluminum-garnet (YAG) fine particles activated by cerium that is a yellow light-emitting fluorescent body are mixed into a hard silicone resin. The housing frame 16 is formed of synthetic resin or metal.
In the illustrated example, an optical sheet 22 is arranged on the light exit face 12a side of the light-guiding plate 12. Furthermore, a reflective sheet 24 is arranged on the back face 12b side as the other principal face of the light-guiding plate 12. A frame-shaped light-shielding sheet 28 is arranged on the light exit face 12a of the light-guiding plate 12 so as to hold the periphery of the optical sheet 22. In the illustrated example, part of an FPC 26 on which the LED 14 is mounted overlaps the light-guiding plate 12 in a plan view, and the overlapped part is caused to adhere to the back face 12b of the light-guiding plate 12 with double-sided adhesive tape 30. A plurality of dome-shaped projections 32 are formed on the back face 12b of the light-guiding plate 12. Furthermore, an uneven structure 34 (a multi-ribbed prism with an arc-sectioned shape, a V-sectioned shape, or the like) linearly extending from the light incident end face 12c toward the facing end face is arranged on the exit face 12a of the light-guiding plate 12 (refer to Japanese Patent Application Laid-open No. 2009-129792, for example).
The dome-shaped projections 32 of the light-guiding plate 12 in FIG. 13 are optical elements functioning as a light exit pattern and cause light incident on the back face 12b of the light-guiding plate 12 out of light incident from the light incident end face 12c and traveling inside the light-guiding plate 12 to be reflected or scattered toward the exit face 12a and to exit from the exit face 12a in a planar manner. The linearly extending uneven structure 34 is an optical element functioning as a light diffusion pattern and diffuses the light incident on the exit face 12a of the light-guiding plate 12 in a direction parallel to the light incident end face 12c and improves the uniformity of illuminating light exiting from the exit face 12a. The linearly extending uneven structure 34 also has a function of forwarding light toward the facing end face 12d while diffusing the light in a direction parallel to the light incident end face 12c so that the light incident from the light incident end face 12c in a plan view of the light-guiding plate 12 will not escape from a pair of side end faces perpendicular to the light incident end face 12c to the outside. By thus providing the optical elements having different shapes and optical functions on the front and back of the light-guiding plate 12, bright illuminating light with the occurrence of brightness unevenness and bright lines reduced can be obtained even with a configuration using the LED 14 as the point light source.
In forming the dome-shaped projections 32 and the linearly extending uneven structure 34 as the optical elements on the principal faces 12a and 12b of the light-guiding plate 12, the optical elements differ in a method of processing. When the light-guiding plate 12 is formed using a mold, for example, as for the dome-shaped projections 32, the mold is irradiated with laser light to form recesses corresponding to the projections. As for the linearly extending uneven structure 34, a corresponding uneven structure is formed on the mold with a cutting tool using a bit or the like. At least two kinds of methods of processing are thus required to be used for the manufacture of a mold for forming one light-guiding plate 12, and consequently, mold processing costs increase, which hinders cost reduction in planar lighting devices. Cutting using a bit in particular tends to take a longer processing time, which promotes an increase in processing costs. The same further holds true for a case of directly forming the dome-shaped projections 32 and the uneven structure 34 on a light-guiding plate (a transparent substrate).
There is a need for a planar lighting device that is excellent in the uniformity of brightness at low cost by providing a light-guiding plate included in the planar lighting device with characteristic optical elements.